The present invention relates to a stabilized fluorine-containing polymer and a method for stabilizing a fluorine-containing polymer. In particular, the present invention relates to a stabilized fluorine-containing polymer which can prevent brown or grayish brown coloration of a fluorine-containing polymer having a terminal carboxylic acid group and/or a terminal group that generates the terminal carboxylic acid group on heating, and avoid foaming, and a method for stabilizing a fluorine-containing polymer to obtain such a stabilized fluorine-containing polymer.
For example, when the emulsion polymerized copolymer of tetrafluoroethylene and hexafluoropropylene is melt processed to obtain a final product, the final product may have bubbles or voids formed with volatile materials due to polymer chain ends which are unstable against heat and shear force.
The kinds of unstable polymer chain end groups vary with polymerization methods, and the kinds of polymerization initiators and chain transfer agents. For example, when a commonly used peracid salt (e.g. ammonium persulfate, potassium persulfate, etc.) is used as a polymerization initiator in the emulsion polymerization, terminal carboxylic acid groups are formed, and function as the source of volatile materials in the course of melt processing. In some cases, olefinic groups (xe2x80x94CFxe2x95x90CF2), acid fluoride groups (xe2x80x94COF), etc. may be formed at the polymer chain ends depending on the condition of the melt processing. Such terminal groups may also cause bubbles or voids in the final products.
To remove such unstable terminal groups, U.S. Pat. No. 3,085,083 proposes the wet thermal treatment. However, the wet thermal treatment accelerates the treating speed by the addition of a salt or a base, and the concentration of the salt or the base is from 100 to 600 ppm. However, such a concentration is not set to aim at the unstable terminal group.
For example, the copolymer of tetrafluoroethylene and hexafluoropropylene prepared by the emulsion polymerization may be colored brown or grayish brown during the melt processing to obtain the final products. The causes for such coloration include the severity of the melting conditions, the residues of the polymerization initiators, the presence of contamination, carbonization of low molecular weight materials, etc.
The above-described problems of the formation of bubbles or voids and the coloration can be solved by the fluorination described in U.S. Pat. No. 4,626,587. However, when fluorine is used for the fluorination, some drawbacks such as the increase of the costs and the large scale measures to prevent environmental contamination are not avoided.
One object of the present invention is to provide a stabilized fluorine-containing polymer from which unstable terminal groups are removed, and which is not bubbled on heating.
Another object of the present invention is to provide a method which can effectively remove the unstable terminal group and the coloration caused by the unstable terminal groups from a fluorine-containing polymer having unstable terminal groups such as terminal carboxylic acid groups, and optionally remove unstable bonds in the backbones of such a polymer.
According to the first aspect of the present invention, there is provided a melt formed fluorine-containing polymer containing 1.0xc3x971014 spins/g or less of unpaired electrons on the carbon atoms in terms of a spin density measured with electron spin resonance at a temperature of 10K, and preferably containing 50 ppm or less of a basic component.
According to the second aspect of the present invention, there is provided a method for stabilizing a fluorine-containing polymer having terminal carboxylic acid groups or terminal groups that generate the terminal carboxylic acid groups on heating at a temperature of at least 200xc2x0 C. comprising the steps of:
adding 0.1 to 10% of a compound comprising an alkali metal or an alkaline earth metal in terms of the number of atoms of the alkali metal or the alkaline earth metal based on the total number of the above terminal groups, 0.1 to 10% of ammonia in terms of the number of ammonia molecules based on the total number of the above terminal groups, or 0.1 to 10% of a compound having an ammonium group in terms of the number of the ammonium groups based on the total number of the above terminal groups to the fluorine-containing polymer, and
heating the fluorine-containing polymer at a temperature of at least 200xc2x0 C. in an atmosphere containing moisture.